Movement disorders are neurologic syndromes characterized by either an excess or a paucity of movement. These disorders affect approximately two million Americans, including over one million suffering from benign essential tremor, and half a million suffering from Parkinson's Disease. A substantial percentage of those afflicted with movement disorders experience a significant decrease in quality of life, suffering such problems as incapacitating tremor, limited mobility, bradykinesia (difficulty consciously initiating movement), dysarthria (difficulty with speech), and consequent social isolation. The etiology of many movement disorders, e.g., benign essential tremor, is poorly understood. For other movement disorders, e.g., Parkinson's disease, the mechanism of the disorder and even the brain cells affected have been identified, but even with optimal care the disease may not be reversed and may even continue to progress.
Parkinson's Disease is caused by a gradual loss of dopaminergic (i.e., dopamine-secreting) neurons in the substantia nigra. Consequently, levels of dopamine decrease in the striatum (i.e., the putamen and the caudate nucleus). Although dopamine has both excitatory and inhibitory effects on the striatum, the predominant effect of the loss of dopamine is decreased inhibition (by GABA) of the internal segment of the globus pallidus. This leads to increased GABA output from the internal segment of the globus pallidus, which inhibits the ventrolateral thalamus. This leads in turn to decreased inhibition of (and ultimately decreased control over) the motor cortex. The subthalamic nucleus appears to increase its activity in Parkinson's Disease as well, and this is believed to contribute to the symptoms of the disease.
Essential Tremor (ET), a.k.a. Benign Essential Tremor, is the most common movement disorder. It is a syndrome characterized by a slowly progressive postural and/or kinetic tremor, usually affecting both upper extremities. The prevalence of ET in the US is estimated at 0.3–5.6% of the general population. A 45-year study of ET in Rochester, Minnesota reported an age- and gender-adjusted prevalence of 305.6 per 100,000 and an incidence of incidence of 23.7 per 100,000.
ET affects both sexes equally. The prevalence of ET increases with age. There are bimodal peaks of onset—one in late adolescence to early adulthood and a second peak in older adulthood. The mean age at presentation is 35–45 years. ET usually presents by 65 years of age and virtually always by 70 years. Tremor amplitude slowly increases over time. Tremor frequency decreases with increasing age. An 8–12 Hz tremor is seen in young adults and a 6–8 Hz tremor is seen in the elderly. Although ET is progressive, no association has been found between age of onset and severity of disability.
Mortality rates are not increased in ET. However, disability from ET is common. Significant changes in livelihood and socializing are reported by 85% of individuals with ET, and 15% report being seriously disabled due to ET. Decreased quality of life results from both loss of function and embarrassment. In a study of hereditary ET, 60% did not seek employment; 25% changed jobs or took early retirement; 65% did not dine out; 30% did not attend parties, shop alone, partake of a favorite hobby or sport, or use public transportation; and 20% stopped driving.
There are no known pathological findings associated with ET. However, it has been hypothesized that ET is the result of an abnormally functioning central oscillator that is located in Guillain Mollaret triangle near the brainstem and involves the inferior olivary nucleus. In addition, there is probable involvement of cerebellar-brainstem-thalamic-cortical circuits.
When harmaline, a Monoamine Oxidase (MAO) inhibitor, is administered to primates with ventromedial tegmental tract or lateral cerebellum lesions, an ET-like tremor is produced. In these animals, inferior olivary nucleus neurons fire synchronously at the tremor frequency. Hypermetabolism has also been demonstrated in the inferior olivary nuclei of rats and cats with harmaline-induced tremor.
In patients with ET, studies have identified increased glucose consumption in the medulla. An increase in medullary regional cerebral blood flow in subjects with ET occurred only after administration of ethanol, and showed bilateral overactivity of cerebellar circuitry.
Patients suffering from tremor (e.g., due to ET or Parkinson's disease) and other symptoms may undergo surgery to lesion a part of the brain (e.g., the ventral intermediate (Vim) nucleus of the thalamus, the internal globus pallidus (Gpi), or the subthalamic nucleus (STN)), which may afford some relief. However, lesions are irreversible, and may lead to side effects such as dysarthria or cognitive disturbances. Additionally, lesions generally yield effects on only one side of the body (the contra-lateral side), and bilateral lesions are significantly more likely to produce side effects. Other surgical procedures, such as fetal tissue transplants, are costly and unproven.
High frequency chronic electrical stimulation (i.e., frequencies above about 50–100 Hz) of certain areas of the brain has been demonstrated to be as efficacious as producing a lesion in any one of those areas. In contrast to ablation surgery, chronic electrical stimulation is reversible. Additionally, stimulation parameters may be adjusted to minimize side effects while maintaining efficacy; such “fine tuning” is unavailable when producing a lesion. An implantable chronic stimulation device for deep brain stimulation (DBS) is available and similar systems are under development.
Vagus nerve stimulation (VNS) has been applied with partial success to patients with refractory epilepsy. In this procedure, an implantable pulse generator in implanted in the patient's thorax, and an electrode lead is routed from the IPG to the left vagus nerve in the neck. Helix-shaped stimulation and indifferent electrodes are attached to the vagus nerve via an invasive surgical process that requires the carotid sheath to be fully exposed. Based on a number of studies, approximately 5% of patients undergoing VNS are seizure-free, and an additional 30–40% of patients have a greater than 50% reduction in seizure frequency. However, VNS may lead to significant side effects. The vagus nerve provides parasympathetic innervation to the cardiac tissue, and thus VNS may lead to bradycardia, arrhythmia, or even graver cardiac side effects. In fact, VNS systems are only used on the left vagus nerve, as the right vagus nerve contributes significantly more to cardiac innervation. Additionally, VNS may interfere with proper opening of the vocal cords, which has led to hoarseness and shortness of breath in a significant number of VNS patients.
The exact mechanism of action of VNS is unknown. The nucleus of tractus solitarius (NTS; a.k.a., nucleus of the solitary tract) is a primary site at which vagal afferents terminate. Because afferent vagal nerve stimulation has been demonstrated to have anticonvulsant effects, it is likely that changes in synaptic transmission in the NTS can regulate seizure susceptibility. To demonstrate this, Walker et al. applied muscimol, an agonist of the inhibitory neurotransmitter GABA, to the NTS in a murine model of epilepsy [“Regulation of limbic motor seizures by GABA and glutamate transmission in nucleus tractus solitarius,” Epilepsia, 1999 August]. Muscimol applied to the NTS attenuated seizures in all seizure models tested, whereas muscimol applied to adjacent regions of NTS had no effect. Additionally, bicuculline methiodide, a GABA antagonist, injected into the NTS did not alter seizure responses. Finally, anticonvulsant effects were also obtained with application of lidocaine, a local anesthetic, into the NTS. Unilateral injections were sufficient to afford seizure protection. Walker, et al. concludes that inhibition of the NTS outputs enhances seizure resistance in the forebrain and provides a potential mechanism for the seizure protection obtained with vagal stimulation.
The NTS sends fibers bilaterally to the reticular formation and hypothalamus, which are important in the reflex control of cardiovascular, respiratory, and gastrointestinal functions. The NTS also provides input to the dorsal motor nucleus of the vagus, which enables the parasympathetic fibers of the vagus nerve to control these reflex responses. The NTS runs the entire length of the medulla oblongata, and the NTS (as well as the trigeminal nucleus) receives somatic sensory input from all cranial nerves, with much of its input coming from the vagus nerve. In 2001, Handforth et al. studied whether VNS could suppress tremor in the harmaline tremor model in the rat [Handforth et al. “Suppression of harmaline-induced tremor in rats by vagus nerve stimulation.” Movement Disorders, 2001 Jan; 16(1): 84–8]. Animals were chronically implanted with helical leads around the left vagus nerve and a disk-shaped electrode positioned subcutaneously in to dorsal neck. Harmaline-induced tremor was recorded on a physiograph while each animal received a sequence of five 20-minute trials. Each trial consisted of five minutes of pro-stimulation baseline, five minutes of VNS, and ten minutes of post-stimulation. VNS significantly suppressed harmaline-induced tremor. The suppressive effect was present within the first minute of stimulation and was reproducible across the five trials of the study. The results of this study suggest that the central generator or expression of tremor in the harmaline animal model can be suppressed by VNS. This further suggests that VNS may be an effective therapy for ET and/or other movement disorders.
Additional and improved treatment options are needed for patients suffering from movement disorders.